The present invention relates generally to endless power transmission belts having excellent high temperature durability as well as significantly improved low temperature crack resistance. More particularly, the invention relates to endless power transmission belts and process therefor, manufactured from a fiber reinforced nitrile group-containing, highly saturated copolymer rubber, which exhibit a good balance of high temperature resistance, good belt durability and low temperature crack resistance. The invention furthermore relates to synchronous belts and frictional power transmission belts, manufactured from a fiber reinforced nitrile group-containing, highly saturated copolymer rubber, which exhibit good belt durability and a service range of from about xe2x88x9240xc2x0 C. to about 140xc2x0 C.
Power transmission belts used with toothed pulleys (or sprockets or sheaves) are well known in the art. The most widely used of these toothed belts are so-called synchronous or positive drive belts. It is well known to use a toothed belt to provide synchronization between two rotating shafts, the belt comprising a back surface section, a plurality of teeth spaced apart and disposed opposite of said back surface section, whereby a tooth land is formed between two adjacent teeth, a tensile layer contacting and interposed between said back surface section and said plurality of teeth and preferably a covering for each of said teeth and each of said tooth lands, wherein the back surface and the plurality of teeth are made of an elastomeric material. Certain applications, exemplified by automotive applications, place high demands on power transmission belts, including a high degree of durability and a broad service temperature range.
In operation, the power transmission toothed belt is subjected to the most stress at the bottom of each tooth when meshed with complementary pulley teeth to transmit power. Since this stress is substantially sustained by the elastomeric material, the material beneficially possesses a high modulus so that the toothed belt can withstand a high load. It is known to increase the amount of filler, e.g., carbon black, in the belt elastomer to increase the cured rubber""s modulus. Increasing filler content however, is known to adversely impact belt performance at both high- and low temperatures. Both poor high temperature aging resistance and poor low temperature capability in a synchronous belt manifest themselves in the formation of cracks in the back surface section.
It has also been suggested to incorporate a fiber reinforcement means into the matrix of the belt""s elastomeric components to increase the shear strength of the teeth. There has been some criticism as to the practical application of this technique with respect to its impact upon the tensile strength of the belt. Synchronous belts have typically been made by one of three methods: the extruded tooth method, as described by Case in U.S. Pat. No. 2,507,852, the tooth pre-form method as described by Geist et al. in U.S. Pat. No. 3,250,653 or the flow-through method as described by Skura in U.S. Pat. No. 3,078,206. With respect to fiber loading of the belt elastomer, it has been noted that in practice, fiber reinforced toothed belts prepared by the flow-through method of Skura exhibit decreased tensile strength because the tensile members must be spaced further apart than in non-fiber-loaded elastomer belts, to allow the fiber-filled elastomeric material to flow through the tensile members.
As noted above, power transmission belts exemplified by automotive synchronous belts are generally required to operate at increasingly low and high temperatures. Synchronous belts may be used, for example, for driving the overhead camshaft of an automobile. It is not unusual for the operating temperature of the belt to reach 140xc2x0 C. in such applications. The elastomeric material used for the back surface and plurality of teeth becomes vulnerable to heat aging in such harsh environments, which can give rise to severe cracking and premature failure.
It has been suggested to load the elastomeric material of the belt with certain types of fiber to improve the high temperature resistance thereof. Adding fiber to the uncured elastomer however, has the effect of increasing both the viscosity of the uncured material, and the modulus, i.e., hardness or stiffness, of the material in the cured state. In cold weather climates, ambient temperatures can reach xe2x88x9240xc2x0 C. or lower. The higher the modulus and hardness of an elastomeric material however, the poorer its low temperature flexibility and crack resistance.
U.S. Pat. Nos. 5,250,010 to Mishima et al. and 5,254,050 to Nakajima et al. show the experimental tests run on V-ribbed belts to measure heat resistance and low temperature resistance range from xe2x88x9230xc2x0 C. to 130xc2x0 C. and xe2x88x9230xc2x0 C. to 120xc2x0 C., respectively, but do not disclose a power transmission belt having good durability over a service range of from about xe2x88x9240xc2x0 C. to about 140xc2x0 C. High temperature resistance and good load carrying capability can be obtained in a synchronous belt having elastomeric portions made of a conventional hydrogenated nitrile-butadiene rubber (HNBR) copolymer (incorporating two monomers only). However, synchronous belts made of such HNBR copolymers have not been known to exhibit good low temperature flexibility or crack resistance below xe2x88x9230xc2x0 C. or xe2x88x9235xc2x0 C.
The need remains, particularly in the area of synchronous and frictional power transmission belts formed of rubber elastomer, for a power transmission belt that exhibits a good balance of high temperature resistance, good belt durability and low temperature crack resistance.
The present invention provides a power transmission belt adapted to engage a sheave or pulley, comprising a main belt body portion; a pulley- or sheave-contact portion integral with the body portion, and a tensile member disposed in the body portion. At least one of the main belt body portion and the pulley contact portion comprises an elastomeric material comprising 100 parts by weight of an at least partially hydrogenated nitrile group-containing copolymer rubber, and from about 0.5 to about 50 parts per hundred weight of said nitrile group-containing copolymer rubber of a fiber reinforcement material. According to a preferred embodiment, the at least partially hydrogenated nitrile group-containing copolymer rubber comprises (1) from about 5 to about 40 percent of unsaturated nitrile monomer units, (2) from about 1 to about 80 percent of units from at least one second monomer which possesses the characteristic of lowering the glass transition temperature of the rubber, (3) up to about 20 percent of conjugated diene monomer units and (4) the balance being hydrogenated conjugated diene monomer units. In a preferred embodiment, the sum of the contents of the monomer units (1) and (2) is from about 30 to 90 percent by weight and the sum of the contents of the monomer units (3) and (4) is from about 10 to about 70 percent by weight. A process for forming such belt is furthermore provided.
The use of the fiber reinforcement in the at-least-partially hydrogenated nitrile group-containing copolymer rubber according to a preferred embodiment of the invention has unexpectedly been found to provide substantially improved low temperature crack resistance while improving the high temperature resistance of the belt compared to conventional belts. The invention furthermore provides a power transmission belt with a service range of about xe2x88x9240xc2x0 C. to about 140xc2x0 C. or even higher, as well as good belt durability.